A retrofit kit and assembly for establishing closed loop water supply to a humidifier associated with a furnace. A reservoir defining housing is adapted to being mounted to an external location of the furnace. A fluid supply communicates to the housing and can include either of a float valve actuated city water inlet line or a water condensate line extending from the furnace. An outlet extends from a location of the housing to the humidifier. A return line extends from the humidifier to a further location of the housing. An overflow drain extends from another location of the housing and communicates any excess volume of water to a floor drain.